The present invention relates to a low pressure lubrication system for a gear pump of the type wherein the suction port is opened in the side wall of the cover or mounting plate securely attached to the casing having a pump chamber and more particularly a low pressure lubrication and cooling system for a gear pump wherein part of the liquid drawn and flowing through a low pressure passage which is first extended at right angles to the axes of the intermeshing gears towards one ends of the shafts or trunions thereof and then bent to extend in parallel with the axes of the gears, is circulated to lubricate and cool the rubbing surfaces of the shafts and their bearings.
The same inventor has disclosed the low pressure lubrication system in U.S. Pat. No. 4,090,820. This system is applied to a gear pump of the type wherein both the suction and discharge ports are extended through the casing with the pump chamber at right angles to the axes of the meshing gears. Liquid under low pressure drawn through the suction port is used for lubricating and cooling the shafts of the gears.
In the case of the gear pumps with the suction port formed in the side wall of the cover or mounting plate, the liquid drawn through the suction port is redirected to flow through a low-pressure passage extending in parallel with the axes of the gears, into the spaces between the teeth of the gears. With the low pressure lubrication system of the type disclosed in the above patent, it is therefore extremely difficult to direct the liquid under low pressure especially towards the shafts of the gears at the upstream of the liquid because of the reasons to be described below. First, the above lubrication system utilizes the kinetic energy of liquid under low pressure which is drawn through the suction port and impinges against the teeth of the gears, in order to force part of the liquid into the low pressure chambers. In the case of the gear pumps to which the present invention is applied, the liquid under low pressure flows in parallel with the teeth of the gears and is trapped in the spaces between the teeth. Therefore when the above low pressure lubrication system is applied to these gear pumps, the low pressure chambers at the upstream of the liquid under low pressure are opened toward the downstream so that it becomes impossible to force part of the liquid under pressure into the low pressure chambers at the upstream by the use of the kinetic energy of the liquid under low pressure.
In order to apply the above lubrication system, the low pressure passage must be further bent at right angles so that the liquid under low pressure may impinge forcefully against the tooth surfaces. However, this arrangement would result in the complicated structure and the increase in resistance to the flow of liquid under low pressure. As a result, the pumping capacity would be considerably decreased as compared with the gear pumps of comparable sizes.